Posted 23 February 2012 - 04:46 PM
I have seen this happen a couple times before with my clients. Typically one of two scenarios is happening.
Firstly, somehow your account password has become compromised and they spammer is logging in and trying to send mail directly from your account or secondly they are spoofing your email, meaning that they are sending spam out from another server somewhere and using your email address as the address it is coming from hence when the messages fail and the bounce back is sent, you receive it in your mailbox.
I would change your email password just to be on the safe side and usually the failed emails will stop after a few days.
Also, I would suggest you do a full virus scan of your computer as well as scan your computer for any malware.